CN113073485B - Preparation method of nano cellulose fiber and product thereof - Google Patents
Preparation method of nano cellulose fiber and product thereof Download PDFInfo
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Abstract
The invention relates to the technical field of nano-cellulose, in particular to a preparation method of nano-cellulose fiber and a product thereof; specifically, crop straws are used as raw materials, steam flash explosion treatment is carried out on the raw materials, then high-pressure homogenization is carried out, and then lignin and hemicellulose are removed to obtain the nano cellulose fibers. The data obtained by the Box-Behnken experiment are processed by Design-expert.V8.0.6 software, and the optimal steam flash explosion conditions are obtained as follows: the steam flash explosion pressure value is as follows: 1.52MPa, and the steam flash explosion time is as follows: 283.31s, sodium hydroxide concentration: 1.59 wt%, the nanocellulose yield obtained under these conditions was: 46.02 percent. The invention provides reference for optimizing steam flash explosion process parameters and improving the preparation and research of the yield of the nano-cellulose, and lays a foundation for further research of the nano-cellulose.
Description
Technical Field
The invention relates to the technical field of nano-cellulose, in particular to a preparation method of nano-cellulose fiber and a product thereof.
Background
Nanocellulose refers to cellulose crystals in which at least one dimension of cellulose in the nanomaterial is in the nanometer scale (1-100nm), and the nanomaterial is dispersed in water to form a stable suspension. According to the morphology, particle size and source of nanocellulose, nanocellulose is roughly classified into the following 3 types: cellulose Microfibrils (MFC), nanocellulose whisker, Bacterial nanocelluloses (BC). The preparation of nanocellulose by using plants as raw materials can be divided into two types according to crystal forms: nano-or microfibrillated cellulose (NFC) and Nanocrystalline cellulose (CNC). Compared with a CNC short rod-shaped structure, the NFC has the advantages of large length-diameter ratio and specific surface area, high crystallinity, good hydrophilicity, strong space expansibility, surface hydroxyl activated, easy surface chemical modification, and is a novel nano material, and has good application prospect in the fields of food processing, medical drugs and the like.
Through investigation, the corn straws produced by China every year are about 1.7 hundred million tons, and the development momentum of the planting industry is rapid, so that a large amount of straws are accumulated, people only need to burn the straws, and a large amount of PM2.5 is generated by the combustion of the large amount of straws, so that the environment pollution of a large area is caused. How to reasonably utilize the corn straw resource becomes a big problem facing people at present. The corn stalks contain abundant cellulose, hemicellulose and lignin, wherein the content of crude fiber is as high as 31 to 41 percent. Therefore, it would be highly economic if a method for preparing Nanocellulose Fibers (NFC) from crop straws could be provided.
Disclosure of Invention
Based on the content, the invention provides a preparation method of nano-cellulose fibers and a product thereof.
According to one technical scheme, the preparation method of the nano-cellulose fiber is characterized in that crop straws are used as raw materials, high-pressure homogenization is carried out after steam flash explosion treatment, and then lignin and hemicellulose are removed to obtain the nano-cellulose fiber.
Steam flash explosion is a technology for separating substances by utilizing high-temperature and high-pressure water vapor to release pressure instantly, and is divided into 4 stages in the process of steam flash explosion: acid hydrolysis and thermal degradation; (II) mechanical fracture action; (III) hydrogen bond disruption; (IV) structural rearrangement. The invention utilizes steam flash explosion to treat crop straws, which destroys the structure of lignocellulose through high-strength acting force, so that cellulose, hemicellulose and lignin are better separated, thereby being beneficial to the extraction of nano cellulose fibers.
Further, the crop straws are soaked in a sodium hydroxide solution before being subjected to steam flash explosion.
Cellulose in the straw raw material treated by the sodium hydroxide is not obviously improved, but the content of hemicellulose and lignin is obviously changed, because the hemicellulose in the straw and the alkali liquor are subjected to peeling reaction, the cellulose glycoside bond in the straw is hydrolyzed and cracked, the acetyl on the hemicellulose molecule is also easy to fall off, and phenolic hydroxyl in the lignin molecular structure is cracked when reacting with NaOH, so that the solubility of the alkali liquor is increased, and the reaction is efficiently and orderly carried out. And the alkali treatment is carried out before the steam flash explosion, so that the cellulose is subjected to swelling action under the condition of high-temperature alkali treatment, and the separation of hemicellulose and lignin is facilitated.
Further, the concentration of the sodium hydroxide solution is 1.0-2.0 wt%, and the soaking time is 24 h.
Further, the steam flash explosion treatment conditions are as follows: the pressure is 0.5-2.5MPa, and the time is 90-360 s.
Further, the sodium hydroxide solution concentration is 1.59 wt%, and the steam flash explosion pressure value is: 1.52MPa, and the steam flash explosion time is as follows: 283.31s, under the condition, the yield of the nano cellulose fiber is up to 46.02 percent
Further, the method specifically comprises the following steps:
(1) cutting crop straws, cleaning, airing, crushing, sieving by a 80-mesh sieve, soaking in a sodium hydroxide solution, performing steam explosion, filtering, washing and drying, and homogenizing for 6 times at a high pressure of 40MP to obtain a straw sample;
(2) putting the straw sample obtained in the step (1) into an aqueous solution containing sodium chlorite and acetic acid, heating until the solution turns white, filtering, washing with water and drying in the air;
(3) and (3) placing the product obtained in the step (2) in an alkaline solution, standing, heating, washing with water, and airing to obtain the nano cellulose fiber.
The steam flash explosion treatment not only helps to generate more cellulose, but also can remove a certain amount of hemicellulose and lignin, and the steam flash explosion treatment can fibrillate the straw, so that the original intertwined cellulose, hemicellulose and lignin are separated, and the hemicellulose and lignin can be removed more easily in subsequent experiments. The high-pressure homogenization can convert cellulose into nano cellulose with smaller particle size through strong shearing and high-pressure treatment, and the hemicellulose and the lignin are not changed in the process.
Further, the heating temperature in the step (2) is 75 ℃, and sodium chlorite and acetic acid are added every 1h in the heating process.
Further, the alkaline solution in the step (3) is specifically a potassium hydroxide solution with the mass concentration of 6%, and the solution is placed at normal temperature for 8 hours and then is heated in an environment with the temperature of 80 ℃ for 2 hours.
Furthermore, the crop straws are corn straws or wheat straws.
According to the second technical scheme, the nano-cellulose fibers prepared by the preparation method of the nano-cellulose fibers are slender fibers with the diameter of 20-220 nm. The smaller diameter makes the nanocellulose more water soluble.
Compared with the prior art, the invention has the following beneficial effects:
the invention takes crop straws as raw materials, and cellulose with nanometer diameter grade is formed through alkali pretreatment, steam explosion and high-pressure homogenization treatment, and the technical purpose of improving the yield of the nanometer cellulose fiber to the maximum extent is realized. The data obtained by the Box-Behnken experiment are processed by Design-expert.V8.0.6 software, and the optimal steam flash explosion conditions are obtained as follows: the steam flash explosion pressure value is as follows: 1.52MPa, and the steam flash explosion time is as follows: 283.31s, sodium hydroxide concentration: 1.59 wt%, the yield of nanocellulose obtained under these conditions was: 46.02 percent. In order to verify the feasibility of the response surface method and consider the specific feasibility of the experiment in the operation process, the steam flash explosion pressure value is 1.5MPa, and the steam flash explosion time is as follows: 285s, sodium hydroxide concentration: 1.6 percent, performing a verification experiment under the condition, and repeating 3 groups of parallel experiments to obtain the nano-cellulose with the yield of 45.88 percent and the error with the predicted value of 46.02 percent within 1.5 percent, which indicates that the process parameter model obtained by adopting the response surface optimization is reliable and has certain significance for improving the yield of the nano-cellulose. Provides reference for optimizing steam flash explosion process parameters and improving the preparation and research of the yield of the nano-cellulose, and lays a foundation for further research of the nano-cellulose.
Drawings
FIG. 1 is the effect of sodium hydroxide concentration on straw composition in example 1;
FIG. 2 is the effect of steam explosion pressure on straw composition in example 1;
FIG. 3 is the effect of steam flash explosion time on straw composition in example 1;
FIG. 4 is a three-dimensional surface plot and a contour plot of the effect of steam flash-off pressure and steam flash-off time on cellulose content for example 1;
FIG. 5 is a three-dimensional surface plot and a contour plot of the effect of steam flash explosion pressure and sodium hydroxide concentration on cellulose content for example 1;
FIG. 6 is a three-dimensional surface plot and a contour plot of the effect of steam flash off time and sodium hydroxide concentration on cellulose content for example 1;
FIG. 7 is a transmission electron micrograph of the nanocellulose in example 1.
Detailed Description
Reference will now be made in detail to various exemplary embodiments of the invention, the detailed description should not be construed as limiting the invention but as a more detailed description of certain aspects, features and embodiments of the invention.
It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. Further, for numerical ranges in this disclosure, it is understood that each intervening value, between the upper and lower limit of that range, is also specifically disclosed. Every smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in a stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference herein for the purpose of disclosing and describing the methods and/or materials associated with the documents. In case of conflict with any incorporated document, the present specification will control.
It will be apparent to those skilled in the art that various modifications and variations can be made in the specific embodiments of the present disclosure without departing from the scope or spirit of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification. The specification and examples are exemplary only.
As used herein, the terms "comprising," "including," "having," "containing," and the like are open-ended terms that mean including, but not limited to.
The following examples of the invention:
materials and reagents
Corn stover (Kerui 981, obtained from Daqing farmers). Ethylenediaminetetraacetic acid is provided by Ronghong chemical industry, Inc.; acetone is supplied by Fuyu Fine chemical Co., Ltd; sodium dodecyl sulfonate and cetyltrimethylammonium bromide were provided by Kjen chemical Co., Ltd, Shanghai; ethylene glycol ethyl ether was supplied by watson chemical; sodium tetraborate decahydrate was supplied by Shanghai Hakken industries, Inc.;
apparatus and device
The PHS.2C type precision pH meter is from METTLER TOLEDO, USA; type FZ102 micro plant grinder from Tianjin Tester; model AB204-N analytical balance from Shanghai Merle-Torledo instruments, Inc.; TDZ5-WS type multi-rack automatic balancing centrifuge is from Changshan instrument centrifuge, Inc.; the SENCO constant-temperature water bath kettle is from Shanghai Shensheng science and technology limited; the QBS-200B type steam flash explosion machine is from the research center of steam explosion engineering technology in the city of crane wall; model AS-15 high pressure homogenizers from France Industrial systems, Inc.; JEM-200CX type transmission electron microscope is available from JEOL, Japan.
Example 1
(1) Cutting corn straws, cleaning, airing, crushing, sieving to 80 meshes, soaking in 100mL of sodium hydroxide solution for 24h, performing steam explosion, filtering, washing and drying to obtain a straw sample, and treating in a high-pressure homogenizer to refine fibers and facilitate the next operation.
(2) Removing lignin: adding 130ml of deionized water, 1.2g of sodium chlorite and 1ml of acetic acid into 2g of straw residues, shaking up, sealing and placing in a water bath kettle at 75 ℃ for heating for 1h, adding 1.2g of sodium chlorite and 1ml of acetic acid every 1h until the solution turns white, stopping heating, washing with deionized water until the solution is neutral, and airing [11-12 ].
(3) Removing hemicellulose: and (3) putting the residues into 300ml of 6% KOH solution, standing at normal temperature for 8h, heating in a water bath kettle at 80 ℃ for 2h, washing with deionized water to be neutral, and airing.
Wherein, the steam explosion conditions in the step (1) adopt a single-factor experiment, which is specifically as follows:
the pressure is maintained at 2.0MPa, the blasting time is 270s, and the mass fractions of the sodium hydroxide concentrations are 0.5%, 1.0%, 1.5%, 2.0% and 2.5%;
and blasting after 0.5MPa, 1.0MPa, 1.5MPa, 2.0MPa,2.5MPa and a downward pressure of 270 s;
and respectively processing the straw raw materials for 90s,180s,270s and 360s under the condition that the pressure is maintained at 2.0 MPa.
Effect verification
1. The content of cellulose, hemicellulose and lignin after different treatments and steam explosion in the embodiment is analyzed, and the analysis method comprises the following steps:
1.1 preparation of reagents
Preparation of a neutral reagent: weighing 18.61g of ethylenediamine tetraacetic acid and 6.81g of sodium tetraborate decahydrate, mixing, adding 250mL of distilled water, dissolving (marked as solution 1) in a water bath, adding 200mL of distilled water into 30g of sodium dodecyl benzene sulfonate, gradually adding 10mL of ethylene glycol ethyl ether and 4g of NaOH, placing on the water bath, heating until the mixture is dissolved (marked as solution 2), adding 150mL of distilled water into 4.56g of sodium dihydrogen phosphate, placing on the water bath, heating (marked as solution 3), mixing the solution 1, the solution 2 and the solution 3, adjusting the pH value to be within the range of 6.9-7.1, and adding water to fix the volume to 1000 mL.
Preparation of an acidic reagent: 20g of hexadecyl trimethyl ammonium bromide is taken and is made into 1000mL by 1mol/L sulfuric acid.
1.2 determination of cellulose, hemicellulose and lignin contents
The content of cellulose and hemicellulose is measured by a paradigm fiber measuring method.
Washing the above straw powder with water in 80 deg.C water bath for 1 hr, oven drying in 60 deg.C oven, placing 1g straw powder in a flat-bottomed flask (marked as m), and adding 0.5g anhydrous sodium sulfiteHeating sodium sulfate, 100mL neutral reagent and several drops of acetone in an electric furnace, boiling, timing, heating and refluxing for 1h, vacuum filtering, washing with boiling water for at least 2 times (30 mL distilled water for each time), washing with acetone for at least 2 times (20 mL acetone for each time), and oven drying the obtained straw powder in an oven at 80 deg.C (m is recorded as 1 )。
Drying the obtained powder (m) 1 ) Placing in a flat-bottomed flask, adding 100mL of acidic reagent and several drops of isooctanol, heating and refluxing, timing after boiling for not less than 1h, performing suction filtration while hot, washing with boiling water for 2 times, washing with acetone for 2 times, collecting all residues, and oven drying (marked as m) 2 ). Mixing straw powder (m) 2 ) Placing in 5mL 75% sulfuric acid, placing in 50 deg.C water oscillator for hydrolysis for 12 hr, hot filtering, washing with boiling water for 2 times, washing with acetone for 2 times, and oven drying to obtain straw powder (denoted as m) 3 )。
The calculation method of hemicellulose and cellulose is as follows:
hemicellulose (%) ═ m 1 -m 2 )/m×100％ (1)；
Cellulose (%) ═ m 2 -m 3 )/m×100％ (2)；
The determination of the lignin content is performed by using the Klason method, which is specifically the prior art and is not described herein.
1.3 analysis of results
(1) Effect of sodium hydroxide concentration
As shown in figure 1, the content of hemicellulose and lignin in the straw raw material treated by sodium hydroxide is not significantly improved, but the content of hemicellulose and lignin is significantly changed because the hemicellulose in the straw and the alkali liquor undergo peeling reaction, the glycoside bond of the cellulose is hydrolyzed and cracked, the acetyl on the hemicellulose molecule is also easy to fall off, and the phenolic hydroxyl in the lignin molecular structure is cracked when reacting with NaOH, so that the solubility of the alkali liquor is increased, and the reaction is efficiently and orderly carried out. And the alkali treatment is carried out before the steam flash explosion, so that the cellulose is subjected to swelling action under the condition of high-temperature alkali treatment, and the separation of hemicellulose and lignin is facilitated.
(2) Influence of steam explosion pressure on straw composition
As can be seen from fig. 2, the content of cellulose after the steam flash explosion treatment is significantly increased, the content of hemicellulose and lignin is decreased, and as the pressure is increased, the content of cellulose is increased and the content of hemicellulose and lignin is decreased. When the steam pressure is 2.5MPa, the cellulose content is increased from 32.65% to 39.14%, the hemicellulose content is decreased from 27.16% to 15.32%, the lignin content is decreased from 18.21% to 12.43%, and the hemicellulose and lignin contents are decreased by 53.45%. When the steam pressure is 1.5MPa, only 2.34 percent of hemicellulose in the straws is degraded, and when the steam pressure is increased to 2.0MPa, the content of the hemicellulose begins to be obviously reduced by 3.83 percent. The reason for this result is that the excessive temperature makes the straw fiber soften more, and part of the cellulose is decomposed at high temperature; it is also possible that when the steam flash explosion pressure is increased, a larger mechanical force is generated in the explosion cavity due to the increase of the temperature of the steam, the hemicellulose is hydrolyzed into acidic substances and dissolved in water due to the action of high temperature and high pressure, partial lignin is degraded and dissolved in water, but a small amount of incompletely decomposed hemicellulose and impurity components of lignin cell walls cannot be dissolved in water, so that the content of the hemicellulose and the lignin cannot be continuously reduced even if the steam pressure is increased.
(3) Influence of steam flash explosion time on straw composition
As can be seen from FIG. 3, as the steam explosion time is prolonged, the content of cellulose in the straw is gradually increased, and the content of hemicellulose and lignin is gradually reduced. The time of steam flash explosion is from 90s to 350s, the cellulose content is increased from 32.03% to 38.01% and is increased by 5.98%, and the content of hemicellulose and lignin is respectively reduced from 27.7% and 20.68% to 15.84% and 12.25% and is respectively reduced by 11.86% and 8.43%. The reason is that in the process of steam flash explosion, the hydrolysis time of hemicellulose is more sufficient along with the prolonging of time, and an acidic environment is formed after the hydrolysis of the hemicellulose to be favorable for the decomposition of lignin.
1.4 optimization of the response surface Condition
According to the center combination experiment design principle of Box-Behnken, based on a single-factor test result, a steam flash explosion pressure value (A), steam flash explosion time (B) and sodium hydroxide concentration (C) are used as response factors, cellulose content (Y) is used as a response value, a response surface analysis method with 3 factors and 3 levels is adopted to carry out experiment design, and the factors and the coding level are shown in Table 1. And processing data and analyzing a response surface by using Design-expert.V8.0.6 software.
TABLE 1
17 test groups with three factors and three levels are designed according to the Box-Behnken principle to carry out response surface optimization analysis tests, and the design and the results of the response surface tests are shown in a table 2. Performing regression fitting analysis on the response value and the coding value of each factor by using Design-expert.V8.0.6 software to obtain a quadratic multiple regression equation
Y＝45.92+0.7663A+0.6738B+0.3500C-1.46AB-1.53AC+1.46BC-4.33A 2 -2.96B 2 -1.49C 2 Results of analysis of variance on the regression equation are shown in table 3. As can be seen from table 3, the significance (P ═ 0.0001) of the regression equation model established for the corn stalk nanocellulose is extremely high, and the mismatching term (P ═ 0.0652)>0.05) no significance, adjustment coefficient of model R 2 ＝0.9715，R 2 Adj The result of 0.9348 shows that the model has a good fit with practical experiments, the linear relation between the independent variable and the response surface is significant, and the experimental error is small, so that the regression model can be used for analyzing and predicting the process parameters with the highest cellulose content in the corn straw nanocellulose.
As can be seen from the significance test of the regression equation coefficient in Table 3, the AC and A in the model 2 、B 2 、C 2 The influence on the response surface is extremely remarkable (P)<0.01), the effects of A, C, AB, BC on the response surface were significant (P)<0.05), B, C had insignificant effect on the response surface (P)>0.05). In the regression equation, each factorThe coefficient value of the element directly reflects the influence effect of each test factor and the index value. According to the mean square value of all factors, the influence sequence of all factors on the cellulose content in the corn straw nanocellulose is as follows (steam flash explosion pressure value (A), steam flash explosion time (B) and sodium hydroxide concentration (C)): steam flash explosion pressure value (A)>Steam flash explosion time (B)>Sodium hydroxide concentration (C), the primary and secondary order of interaction between 3 factors is: AC>BC>AB。
TABLE 2
TABLE 3
The interaction of 3 factors of steam flash explosion pressure value, steam flash explosion time and sodium hydroxide concentration in the reaction process is shown in figures 4-6. If the trend of the curve in the response surface is steeper and steeper, the interaction of the two factors is more remarkable, and if the trend of the curve of the response surface is smoother, the interaction influence of the two factors is smaller; the contour lines are elliptical indicating that the two factors interact significantly, while the circles indicate that the interaction is not significant.
As can be seen from fig. 4, the contour lines appear elliptical, indicating that the interaction between the steam explosion pressure and the steam explosion time is significant. When the steam flash explosion pressure is less than 1.5MPa, the contour lines are dense, which shows that the steam flash explosion pressure has obvious influence on the content of the cellulose in the corn straws (p is less than 0.05); when the steam flash explosion pressure is 1.5-2MPa, the curve is stable, the contour lines at the moment are sparse, and the influence on the cellulose content is smaller and smaller along with the continuous increase of the steam flash explosion pressure. As can be seen from FIG. 5, as the trend of the 3D graph changes, the color gradually deepens and the gradient becomes steep, and the interaction of the steam explosion pressure and the sodium hydroxide concentration is obvious (p < 0.01). When the steam flash explosion pressure and the concentration of sodium hydroxide are at 0 level, the cellulose content is a larger value and is kept unchanged; when the steam explosion pressure and the sodium hydroxide concentration are lower than 0 level, the content of the cellulose increases along with the increase of the steam explosion pressure and the sodium hydroxide concentration, and the content shows a more obvious rising trend. As can be seen from FIG. 6, there is a significant interaction between the steam flash explosion time and the sodium hydroxide concentration, the sodium hydroxide concentration is fixed, the cellulose content shows a trend of rapidly increasing and then slowly decreasing with the increase of the steam flash explosion time, and the cellulose content is always decreased with the increase of the sodium hydroxide concentration under the same steam flash explosion time.
The data obtained by the Box-Behnken experiment are processed by Design-expert.V8.0.6 software, and the optimal steam flash explosion conditions are obtained as follows: the steam flash explosion pressure value is as follows: 1.52MPa, and the steam flash explosion time is as follows: 283.31s, sodium hydroxide concentration: 1.59 wt%, the nanocellulose content obtained under these conditions was: 46.02 percent. In order to verify the feasibility of the response surface method and consider the specific feasibility of the experiment in the operation process, the pressure value of the steam flash explosion is 1.5MPa, and the steam flash explosion time is as follows: 285s, sodium hydroxide concentration: 1.6 percent, performing a verification experiment under the condition, and repeating 3 groups of parallel experiments to obtain the nano-cellulose with the content of 45.88 percent and the error with the predicted value of 46.02 percent within 1.5 percent, which indicates that the process parameter model obtained by adopting the response surface optimization is reliable and has certain significance for improving the yield of the nano-cellulose.
2. Micro-topography analysis
Setting the steam flash explosion pressure value at 1.5MPa, and setting the steam flash explosion time as follows: 285s, sodium hydroxide concentration: ultrasonic treatment is carried out on a sample prepared under the condition of 1.6% for 30min, the sample is uniformly dispersed, a proper amount of the sample is placed on a 400-mesh copper net, the sample is observed under a transmission electron microscope after being dried, and a TEM image of the nanocellulose is shown in FIG. 7. As can be seen from fig. 7, the nanocellulose is in the form of elongated fibers with a diameter of 20-220nm, which also indicates that the nanocellulose is formed after steam explosion and high pressure mean treatment, and the smaller diameter leads to better water solubility of the nanocellulose.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included therein.
Claims (4)
1. A preparation method of nano cellulose fiber is characterized in that crop straws are used as raw materials, steam flash explosion treatment is carried out on the raw materials, then high-pressure homogenization is carried out, and then lignin and hemicellulose are removed to obtain the nano cellulose fiber;
the method specifically comprises the following steps:
(1) cutting crop straws, cleaning, airing, crushing, sieving by a 80-mesh sieve, soaking in a sodium hydroxide solution, performing steam explosion, filtering, washing, drying, and homogenizing under high pressure to obtain a straw sample;
(2) putting the straw sample obtained in the step (1) into an aqueous solution containing sodium chlorite and acetic acid, heating until the solution turns white, filtering, washing with water and drying in the air;
(3) placing the product obtained in the step (2) in an alkaline solution, standing, heating, washing with water, and drying to obtain nano cellulose fibers;
the crop straw is corn straw or wheat straw;
the concentration of the sodium hydroxide solution is 1.0-2.0 wt%, and the soaking time is 24 h;
the steam flash explosion treatment conditions are as follows: the pressure is 0.5-2.5MPa, and the time is 90-360 s.
2. The method for preparing nano cellulose fibers according to claim 1, wherein the heating temperature in the step (2) is 75 ℃, and sodium chlorite and acetic acid are added every 1 hour in the heating process.
3. The method for preparing nano cellulose fibers according to claim 1, wherein the alkaline solution in the step (3) is potassium hydroxide solution with a mass concentration of 6%, and the solution is left to stand at normal temperature for 8 hours and then is heated at 80 ℃ for 2 hours.
4. Nanocellulose fibers produced by the method of production of nanocellulose fibers according to any one of claims 1 to 3, wherein said nanocellulose fibers are in the form of elongated fibers having a diameter of 20 to 220 nm.
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